1. Field of the Invention
The present disclosure relates to systems and methods for utilizing a thermoacoustic engine with a positive displacement reciprocating compressor.
2. Background of the Invention
Due to the increasing costs and environmental concerns associated with hydrocarbon-based energy, society has recently shown greater interest in technologies that promote energy efficiency and alternative sources of energy. One technology that shows great promise in both fields is a thermoacoustic prime mover, which converts heat from any source to acoustic energy (i.e., an acoustic pressure wave).
In general, a thermoacoustic engine consists of a hermetically sealed cylinder housing (often referred to as a resonating tube) containing a pressurized noble gas (e.g., helium or argon). Attached to the inner wall of the cylinder housing is the thermoacoustic engine core. Depending on the configuration, the engine core can induce either a standing or traveling pressure wave in the gas medium.
In the standing wave case, the engine core can consist of a stack sandwiched between a hot and cold exchanger. The stack typically is a porous solid spanning both temperature extremes through which gas oscillates. One characteristic of such a stack is that the pores of the stack are similar in size to the thermal penetration depth of the gas. To start the engine, hot and cold sources are applied to the hot and cold exchangers, respectively. The large temperature gradient created between these two exchangers causes the gas in the stack to channel heat from the hot to the cold end (per the Second Law of Thermodynamics). This oscillating expansion and contraction of gas between exchangers is what creates the acoustic pressure wave. The standing wave time phasing characteristics are due to very poor thermal contact between the gas and the stack (e.g., because of large pore size), which allows gas pressure and relative gas displacement oscillations to be in phase with the gas thermal expansion and contraction.
In contrast to a stack-derived thermoacoustic engine core, a traveling wave engine core incorporates a regenerator, which can also be sandwiched between a hot and cold exchanger. The regenerator, just like the stack, is typically a porous solid spanning both temperature extremes through which gas oscillates. However, in this case the pores are usually much smaller than the thermal penetration depth of the gas. The excellent contact between the porous material and the gas provides for more efficient heat transfer. The improved efficiency allows the oscillating gas thermal expansions and contractions to be in phase with the gas pressure and relative gas velocity oscillations. Another differentiating factor is that the regenerator functions as an amplifier of acoustic power. This acoustic power can be provided by a number of devices, including, but not limited to, a torus shaped resonator (see, e.g., U.S. Pat. Nos. 6,032,464 and 6,314,740), and a cascaded stack (see, e.g., U.S. Pat. No. 6,658,862). An alternative means of facilitating traveling wave time phasing with a regenerator is through the use of a bellows (see, e.g., U.S. Pat. No. 7,143,586 B2).
It is also known in the art that the pressure wave of a thermoacoustic prime mover can be used to reciprocate a mass element (e.g., a piston; see Grant, “Investigation of the Physical Characteristics of a Mass Element Resonator”, M.S. Thesis, Naval Postgraduate School, Monterey, Calif., 1992, National Technical Information Service ADA251792). Furthermore, an electrodynamic linear alternator can be used to convert this mechanical energy to electrical energy (see, e.g., U.S. Pat. Nos. 4,623,808 and 5,389,844). While much discussion has focused on using this electrical energy for space probes and to a lesser extent grid power, one application that has greater potential is electrical compression. Unfortunately, for larger scale compression purposes, this configuration is not practical due to the cost, complexity, and the large number of linear alternators needed.
A related field to the linear alternator is the linear motor compressor (see, e.g., U.S. Pat. No. 5,257,915). However, this device exhibits similar shortcomings, such as complexity and cost.
Therefore, it is apparent that there exists a need to generate larger volumes of compression on a more economical and robust scale via thermoacoustics.